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From: Walter Atkinson <walter@advancedpilot.com>
Subject: Re: [LML] Leaning for Taxi
X-Original-Date: Wed, 2 Mar 2005 11:08:03 -0600
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Mike:

The purpose of a run-up (among other things) is to check the health of 
the ignition system.  The more stress the ignition system is placed 
under, the better the test.  In order of value are the following mag 
checks:

Most valuable
1) high power, inflight LOP.
2) high power, inflight ROP.
3) ground run-up LOP (aggressively leaned).
4) ground run-up ROP.
Least valuable

The last one is all but worthless.  It will only tell you that the mag 
is working or whether or not you have an already DEAD plug.   I find 
weak/non-working plugs during the in-flight LOP check that will not 
show as bad for many more hours during a ROP ground check.  As a 
result, I many times do not do run-up mag checks unless I'm coming out 
of maintenance.  I do the in-flight LOP mag check before the descent on 
every flight.  If I find something, I'm about to land and can get it 
fixed.  If I find it working properly, it will darned sure pass a 
run-up mag check, so I don't fool with them.  The only thing I do is 
switch to left and right mags while taxiing out to confirm that both 
mags are on line.  We find 2-3% of NEW plugs will not pass an inflight, 
LOP mag check.  I toss those.

It's very common to have a bad/weak plug that acts up in flight and 
will pass a run-up and mechanic's bomb test.  That's one source of 
engine roughness that's a head-scratcher for the pilot and mechanic.  
Frequently a high power, inflight mag check will find the problem.  We 
have a significant number of pilots now doing inflight mag checks as a 
matter of routine.  They are they guys whose engines run really well! 
<g>

Walter


On Mar 2, 2005, at 10:33 AM, Mike S. Reinath wrote:


<<< I do the runup at this leaned
mixture setting as it is more diagnostic.  >>>

Walter, or anyone else, please explain why it would be more diagnostic 
to runup leaned as you describe.

Mike Reinath
LNC2 at 460 hrs TT
San Jose, CA (RHV)

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